This disclosure relates generally to techniques for processing materials for manufacture of gallium-containing nitride substrates. More specifically, embodiments of the disclosure include techniques for growing large area substrates using a combination of processing techniques. The disclosure can be applied to growing crystals of GaN, AlN, InN, InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic devices, lasers, light emitting diodes, solar cells, photo electrochemical water splitting and hydrogen generation, photo detectors, integrated circuits, and transistors, and others.
Gallium nitride (GaN) based optoelectronic and electronic devices are of tremendous commercial importance. The quality and reliability of these devices, however, is compromised by high defect levels, particularly threading dislocations, grain boundaries, and strain in semiconductor layers of the devices. Dislocations can arise from lattice mismatch of GaN based semiconductor layers to a non-GaN substrate such as sapphire or silicon carbide. Grain boundaries can arise from the coalescence fronts of epitaxially-overgrown layers. Additional defects can arise from thermal expansion mismatch, impurities, and tilt boundaries, depending on the details of the growth method of the layers.
The presence of defects has a deleterious effect on epitaxially-grown layers. Such effect includes compromising electronic device performance. To overcome these defects, techniques have been proposed that require complex, tedious fabrication processes to reduce the concentration and/or impact of the defects. While a substantial number of conventional growth methods for gallium nitride crystals have been proposed, limitations still exist. That is, conventional methods still merit improvement to be cost effective and efficient.
Progress has been made in the growth of large-area c-plane gallium nitride crystals, typically with a (0001) orientation. The large-area c-plane gallium nitride crystals generally come in 2 inch diameter, free-standing (0001) GaN substrates and are generally available commercially. However, for certain applications other crystallographic orientations may be used.
There has been considerable interest in GaN-based devices fabricated on nonpolar and semipolar crystallographic orientations for at least a decade. Much of this interest derives from the reduction or elimination of piezoelectric and strain-related electric fields that can be very large in conventional c-plane GaN-based devices. However, cost-effective manufacturing of devices generally requires relatively large area substrates, for example, larger than 2″, 4″, or 6″. Efforts to grow such substrates heteroepitaxially has generally produced large concentrations of stacking faults, a particular type of extended defect, at least 103-105 cm−1 or even larger. In addition, very low concentrations of threading dislocations are highly desirable, for example, for laser diode lifetimes, and heteroepitaxy of nonpolar or semipolar GaN wafers generally produces dislocation densities of 108-1011 cm−2. Methods for homoepitaxial growth of nonpolar and semipolar wafers are known, for example, growth of thick c-plane boules by HVPE or ammonothermally following by slicing at a transverse or oblique angle with respect to the growth direction, but it is difficult to make large area wafers by such methods.
In addition, metrology of both threading dislocations and stacking faults presents some challenges. Characterization of high-dislocation and/or high-stacking-fault GaN material, for example, grown heteroepitaxially by hydride vapor phase epitaxy (HVPE), has generally relied on transmission electron microscopy (TEM) or photoluminescence (PL). However, TEM, because of the small sampled area, has insufficient sensitivity to quantify dislocation densities below about 108 cm−2 or stacking fault concentrations below about 103 cm−1, and Luminescence-based methods such as PL and CL rely heavily on the relative intensity of the band-edge emission peak, which may be too low in GaN grown by ammonothermal techniques for reliable detection and quantification of dislocations or stacking faults.
From the above, it can be appreciated that techniques for improving crystal growth and crystal characterization are highly desirable.